Hey Gallade!
As mentioned, Glastrier is definitely superior in comparison to Mamoswine. Glastrier has much better bulk, attack and lower speed which can scare off Trick Room users, not to mention the fact that it can potentially be an extremely viable AV mon as PossiblySomeone mentioned. Assuming that the core of the team is Sableye + Dragapult:
The idea seems extremely great as it counters most of the Metagame:
It can K.O 252 HP 60 def. Fini 87% of the time which is great:
Fini has a 57% to 2HKO you with Moonblast after DMAX so watch out:
It will only K.O 252HP Metagross 12% of the time, so bulky Metagrosses might be a threat after they also proc their Weakness Policy.
Glastrier is a serious threat...
Faster Dragapults can also pick up the K.O on you.
Regieleki is simply not a threat.
Incineroar won't be able to do much against you.
So really, your Dragapult does extremely well against Fini, Metagross, Incineroar, Landorus and Regieleki. However, it struggles against Glastrier and other Dragapults. I would suggset Alolan Marowak here, but the decision is really up to you on how you decide to play it out. However, a larger threat to your gimmick is Follow me/Redirection.
Although they are no longer common these days, things will go south if Togekiss gets off Follow me and your Dragapult becomes extremely passive after no WP activation:
188 Atk Dragapult Max Phantasm vs. 236 HP / 196+ Def Togekiss: 78-93 (41 - 48.9%) -- guaranteed 3HKO
Thus, I would recommend having a second Gimmick which can counteract the flaws with Dragapult + Sableye. The best answer to that should be Glastrier, if I'm correct, easily counters most of the above:
Glastrier @ Life Orb
Level: 50
EVs: 252 HP / 252 Atk / 0 Def / 0 SpA / 0 SpD / 0 Spe
Brave Nature
- Icicle Crash
- Heavy Slam
- High Horsepower
- Close Combat
Nature can easily be adjustable whether you want a TR Glastrier or a gaster Glastrier. The choice is up to you. As you look at the moveset, you might wonder Heavy Slam's role here:
252+ Atk Life Orb Glastrier Max Steelspike vs. 252 HP / 0 Def Dynamax Glastrier: 185-218 (44.6 - 52.6%) -- 22.3% chance to 2HKO
Yeah...
+2 252+ Atk Life Orb Glastrier Max Steelspike vs. 252 HP / 0 Def Dynamax Glastrier: 367-432 (88.6 - 104.3%) -- 25% chance to OHKO
Yeah...
Essentially, you can run, say for example (whether you use this mon is completely up to you), Hitmontop, who can Bullet Punch (not Mach because it will do a little too much + Coverage) to proc. WP, get off Intimidate on the opposing Glastrier and gain an advantage.
Against Dragapult, I won't say much because Glastrier can easily K.O it without any problem...
252+ Atk Life Orb Glastrier Max Hailstorm vs. 4 HP / 0 Def Dynamax Dragapult: 437-515 (133.2 - 157%) -- guaranteed OHKO
Thus, four of your mons should be:
Sableye (F) @ Mental Herb
Ability: Prankster
EVs: 252 HP / 60 Def / 198 SpD
Sassy Nature
IVs: 0 Spe
- Fake Out
- Quash
- Shadow Sneak
- Helping Hand
Dragapult @ Weakness Policy
Ability: Clear Body
Level: 50
EVs: 140 HP / 188 Atk / 92 Def / 4 SpD / 84 Spe
Jolly Nature
- Protect
- Fly
- Phantom Force
- Dragon Darts
Glastrier @ Life Orb
Level: 50
EVs: 252 HP / 252 Atk / 0 Def / 0 SpA / 0 SpD / 0 Spe
Brave Nature
- Icicle Crash
- Heavy Slam
- High Horsepower
- Close Combat
And the Proc WP mon of your choice.
The only thing I would say about the team is that it is extremely weak to fairies and that there aren't many counters to them. I don't think they should pose too much of a threat, as the only one that is being used right now is Tapu Fini which doesn't pose much of an offensive presence. Aegislash can be a candidate, but I don't want to completely throw off your team which has won an entire tournament.
Urshifu is a great late game sweeper and I think it is extremely viable.
Lastly, as you mentioned, Regieleki is slightly not living up to the hype due its lackluster of an immediate offensive presence. I would recommend an Intimidate Pivot such as Incineroar. However, the last slot is entirely up to you as well.
Essentially, I recommend Dragapult, Sableye, Glastrier + WP proc. The last two slots should be your late game sweepers, which depends on your WP proc since I wouldn't advocate Hitmontop + Urshifu due to dual fighting. Additionally, you could try making Glastrier 0 speed to allow other Glastriers to hit you with their max knuckles first followed by your Max Steelspike. Doing this can allow you to use Cresselia and set up TR to allow you to KO the opponent next turn.
Hope I helped!